This invention relates to open-loop hydraulic systems comprising a tank, a pump, at least one directional control valve, and a hydraulic motor. In such systems, the operating fluid (which is normally oil) is drawn from the tank by the pump, flows through the pump, at least one directional control valve, and, in certain states of the circuit, the hydraulic motor, and returns to the tank.
In open-loop hydraulic systems, it is common to warm up the oil in the hydraulic system before the system is actually used. However, the warm up is limited to the oil, the pump, and one passage in the directional control valve (or valves, if the system is of the "high pressure carry-over" type). Other components in the circuit, such as motors, actuators, and other valves, are left cold.
When a directional control valve is shifted, the warm oil in the hydraulic system flows through a cold motor or other component, causing thermal shock. The result is nonuniform expansion of the internal parts of the motor or other component. Galling and component failure may occur. Motors manufactured with small running clearance for efficient operation (such as vane motors) are particularly susceptible to damage from thermal shock, and thermal shock has been a recurring problem where vane motors are used on machines subject to cold ambient temperatures, such as some oil well drilling rigs.